In recent years, an improvement in automobile fuel efficiency (car body weight reduction) to regulate the amount of CO2 emission has been required from the viewpoint of global environmental conservation. In addition to this, in order to ensure the safety of occupants at the time of crash, an improvement in safety on the crash properties of automobile bodies has also been required. It is said that weight reduction due to a decrease in sheet thickness by the high strengthening of a steel sheets within the range of not impairing the stiffness is effective in satisfying an automobile body weight reduction and an improvement in safety at the same time. Recently, high strength steel sheet has been actively used for automobile parts. The weight reduction effect increases as the steel sheet used has higher strength. Therefore, in the automobile industry, there is a tendency to use steel sheet having TS of 440 MPa or more as, for example, an inner panel and an outer panel.
On the other hand, most of automobile parts are formed by press forming. Therefore, an automobile steel sheet is required to have excellent press formability. However, a high strength steel sheet has significantly poor formability, in particular deep drawability, as compared with a mild steel sheet. Consequently, demands for a steel sheet satisfying TS 440 MPa, more preferably TS 500 MPa, and further preferably TS 590 MPa and having still better deep drawability in combination have been increased as issues in pursuing automobile weight reduction, and a high strength steel sheet having a high r value, where an average r value≧1.30 on a Lankford value (hereafter referred to as an r value), which is an evaluation indicator of the deep drawability, has been required.
Furthermore, when the average r values are the same, a reduction in the planar anisotropy has also been required for the smaller planar anisotropy contributes to an improvement in the formability.
As how to enhance the strength while it maintains high r value, for example, Patent Literature 1 discloses a method in which, with respect to an ultra low carbon steel sheet, solid-solution hardening elements, e.g., Si, Mn, and P, are added to a base steel allowed to become IF (Interstitial atom free) steel by addition of Ti or Nb.
However, according to such a technology in which the ultra low carbon steel is used as a raw material and a solution hardening element is added, when production of a high strength steel sheet having TS of 440 MPa or more, 500 MPa or more, or 590 MPa or more is intended, the amount of addition of an alloy element increases. For example, if the amount of addition of Si increases, Si is concentrated in the surface during continuous annealing and reacts with very small amount of water vapor in the atmosphere, Si based oxides are formed on the steel sheet surface, the wettability of the coating is made poor and the quality of coating is degraded significantly. Meanwhile, there are problems in that if the amount of addition of P increases, P segregates at grain boundaries to degrade the resistance to cold-work embrittlement, if the amount of addition of Mn increases, the r value decreases and, therefore, if enhancement of strength is intended, the r value decreases.
As how to enhance the strength of a steel sheet, besides the above-described solid-solution hardening method, a transformation strengthening method is mentioned. In general, a dual-phase steel sheet made from soft ferrite and hard martensite has good ductility and excellent strength-ductility balance and further has a feature of low yield strength. Consequently, the press formability is relatively good. However, the r value is low, and the deep drawability is poor. It is said that this is because martensite, which does not contribute to the r value from the viewpoint of crystal orientation, is present and, in addition, solid solution C indispensable to formation of martensite hinders formation of a {111} recrystallization texture effective for increasing the r value.
As for a technology to improve the r value of such a dual-phase steel sheet, for example, Patent Literature 2 discloses a method in which box annealing is performed at a temperature from a recrystallization temperature to an Ac3 transformation point after cold rolling and, subsequently, in order to obtain dual phase, quenching and tempering is performed after heating in temperature of 700° C. to 800° C. Meanwhile, Patent Literature 3 discloses a high strength steel sheet containing a predetermined amount of C, including 3% or more of at least one of bainite, martensite, and austenite in total in the microstructure, and having an average r value of 1.3 or more.
However, both the technologies described in Patent Literatures 2 and 3 are in need of each of the annealing, which enhances the r value by developing a texture through formation of clusters and precipitates of Al and N, and the heat treatment to form the microstructure. In this regard, at the annealing process, box annealing is required and the holding time is 1 hour or more. Therefore, the box annealing is necessary, but the treatment time is long and the number of steps increases as compared with continuous annealing. Consequently, the efficiency and the productivity are very poor, so that the economy is poor from the viewpoint of the production cost. In addition, there are many problems, e.g., frequent occurrences of adhesion between steel sheets, occurrences of temper color, and reduction in the life of a furnace body inner cover, in the production process.
Meanwhile, Patent Literature 4 discloses a technology to improve the r value of a dual-phase steel sheet by optimizing the V content related to the C content. This produces a dual-phase steel sheet by minimizing the amount of solid solution C through precipitation of C in a steel as a V based carbide before recrystallization annealing to increase the r value, performing heating in an α-γ two-phase region to dissolve the V based carbide and concentrate C into γ, and generating martensite in a cooling step thereafter.
However, in the method in which the V based carbide is dissolved during the two-phase annealing, there is apprehension that the mechanical properties fluctuates because of variations in the dissolution rate of V carbide. Therefore, it is necessary that the annealing temperature and the annealing time be controlled with high precision, and there is a problem in the stability in production with an actual facilities.
Also, Patent Literature 5 discloses a technology to ensure compatibility between a high r value and conversion to a dual-phase by performing control in such a way that the C content is within the range of 0.010% to 0.050% and the Nb content and the C content satisfy 0.2≦(Nb/93)/(C/12)≦0.7. In this regard, according to the technology disclosed, solid solution C necessary for forming martensite after annealing is allowed to remain at the stage of hot rolled sheet and, in addition, the r value is increased on the basis of an effect of a grain refinement of hot-rolled microstructures due to addition of Nb and an effect of reducing solid solution C due to precipitation of NbC.
However, it is the element which the Nb has high cost and delays recrystallization of the austenite. Therefore, there is a problem in that a load in hot rolling is high. In addition, NbC precipitated in the hot rolled sheet enhances the deformation resistance during cold rolling. Therefore, a risk of trouble of the production increases and, in addition, there are problems, e.g., reduction in productivity and limitation on the range of producible products. In this regard, according to the research of the present inventors, in the case where the amount of Nb and the amount of C are somewhat large in this technology, the average r value is good, although the planar anisotropy of the r value tends to increase. Consequently, it is an issue to reduce the planar anisotropy of the r value in a high C content region.
Also, Patent Literature 6 discloses a technology to obtain a high strength steel sheet, wherein the average r value≧1.2 is satisfied and the planar anisotropy thereof is reduced by controlling the Nb content and the C content in a steel in such a way that (Nb/93)/(C/12) becomes 0.15 to 0.45, where the C content is within the range of 0.035% to 0.05%, controlling the slab heating temperature to become 1,000° C. to 1,200° C. and satisfy a relational expression in accordance with the amount of C and the amount of Nb, performing cold rolling, and performing slow heating in a high-temperature region after recrystallization, so as to develop a {111} recrystallization texture effectively. In addition, a technology to perform combined addition in such a way that the Nb content, the Ti content, and the V content satisfy {(Nb/93)+(Ti*/48)+(V/51)}/(C/12)=0.15 to 0.45 has been disclosed, where Ti*=Ti—1.5S-3.4 N and Ti*=0 when Ti*≦0.
However, in the technology described in Patent Literature 6, the average r value in the examples is 1.32 at the maximum and the r values are not always good although the C content is a somewhat high 0.035% to 0.05% and, therefore, the average r value ≧1.2 is satisfied. Therefore, it is predicted that application to a part required to have a higher r value is difficult. Meanwhile, the cost is high because 0.05% or more of very expensive Nb is contained to achieve the average r value ≧1.2, and there is a problem in that a load during hot rolling is high because Nb delays recrystallization of austenite remarkably. Also, NbC precipitated in a hot rolled sheet enhances the deformation resistance during cold rolling and, therefore, reduction in productivity, limitation on the range of producible products, and the like become problems.